Question

1 gm of water at a pressure of $1.01\times {10}^5$ Pa is converted into steam without any change of temperature. The volume of 1 g of steam is 1671 cc and the latent heat of evaporation is 540 cal. The change in internal energy due to evaporation of 1 gm of water is

• A. $\approx$ $167cal$
• B. $\approx$ $500cal$
• C. $540cal$
• D. $581cal$

Answer 1

The correct option is B $\approx$ $500cal$

$dW=P$$\Delta V$ = $1.01\times {10}^5[1671-1]\times {10}^{-6}$ joule
= $\frac{1.01\times 167}{4.2}$cal
= 40 cal. nearly
$\Delta Q=mL=1\times 540$,
$\Delta Q=\Delta W+\Delta U$
or $\Delta U$ $=540-40$

$=500cal.$